Selection, operation and practical guidelines for centrifugal pumps

September 29, 2020
Amin Almasi

Amin Almasi is senior rotating equipment consultant in Australia. He is chartered professional engineer from Engineers Australia and IMechE and registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He specializes in rotating equipment, condition monitoring and reliability.

Centrifugal pumps are widely used in industrial and oil & gas applications. Courtesy of Sulzer.[/caption]

Centrifugal pumps are widely used in plants and facilities. They have one or multiple impellers. They are usually drive by electric motors although steam turbine-driven centrifugal pumps have been used in certain applications.

Horizontal pumps are usually more desirable than vertical pump configurations. But in some applications, special requirements such as low weight, compact configuration, and low net positive suction head (NPSH) might result in the selection of vertical pumps.

Centrifugal pumps usually receive less attention than they deserve. Most should have a positive inlet pressure to prevent cavitation, which is caused by lack of enough positive inlet pressure to prevent liquid vaporization.

Different curves

Pump manufacturers have different performance curves for each standard pump. These curves show the head (pressure) vs. flow rate (capacity).There are usually several sets of curves for each pump model for various impellers and speeds.

The top curve is for the largest diameter impeller that can be used in the pump casing. This has the highest head pressure of any of impellers. It also requires the largest driver. The bottom curve shows the smallest diameter impeller for that pump casing. A different set of curves for the same casing and impeller are for different speed.

Note that many pump curves are purely theoretical. The actual pump performance curve should be obtained during a performance test (at the shop or on site). Large multistage pumps may deviate from their theoretical curves.

Shutoff pressure (shutoff head) is also important. It is the maximum pressure a pump can produce. It is the pressure at the left side of the performance curve equivalent to a near zero flow rate. This metric is important for downstream facilities as it is the maximum pressure that can be experienced downstream. The shutoff pressure determines the pressure rating of downstream equipment and piping, particularly the rating of flanges and vessels, and the class of piping required.

Drivers

Most small or medium centrifugal pumps are driven by constant-speed electric motors. Therefore, throttling is the only practical method of regulating the flow and head.

Many pumps use control valves at their discharge for control and operation. The difference between the head developed by the pump and the head required by the system head curve is usually lost energy. The back pressure produced by a control valve (or throttling valve) imposes a variable amount of loss on the system. This can potentially shift the pump to operate away from its Best Efficiency Point (BEP).

Closing the control valve increases control losses and causes the system head curve to slope up more steeply to intersect the head-flow curve at the desired flow rate. Opening the valve decreases losses and causes the system head curve to slope downward and intersect the head-flow curve at a higher flow rate.

Electric motors using variable speed drives (VSDs) are the most common pump drivers. They are common in medium and large pumps and are sometimes applied to small pumps to save energy and lower operational costs.

Steam turbines can also be used as drivers for pumps. They offer high speed, as well as direct-drive and variable-speed capabilities, simple controls, and non-sparking for nonhazardous operation. Single-stage steam turbines have been used to drive pumps, but variable speed drivers are preferable – they can adjust the speed of the pump and other parameters to help the pump operate near BEP.

Pump selection

Many centrifugal pumps fall in 10 kW to 400 kW range. Models above 1 MW are also used in specific applications.

There are many factors to consider in pump selection. Between-bearing pumps (BB) are often specified for relatively large units. As a rough indications, BB pumps are generall;y required above 330 kW for low-speed pumps (<2,000 rpm) and above 190 kW for high speed pumps (around or above 3,600 rpm).

ANSI pumps or standard OEM pumps are common in water, chemical, and some industrial applications. API 610 pumps are often specified for refinery, oil, gas and petrochemical applications. Therefore, it is wise to study API 610 and use it for critical oil & gas applications. But in general, API should be considered for special applications such as when head is above 15 Barg. For example, boiler feed water (BFW) pumps usually have a discharge pressure more than 40 Bargs – sometimes more than 150 Barg. Thus, it is common to use API 610 BB models for BFW pumps. Additionally, API 610 BB pumps are recommended for temperatures above 160ºC, cryogenic applications, and high speed (>3,600 rpm) pumps.

A prime number of impeller vanes is the norm. Pumps with 5, 7, 11, etc., impellers are typical.

In the selection or arrangement of a centrifugal pump, efficiency, required head, flow range, operating. range and recirculation possibilities should be considered to find an optimum condition for each pump. There are examples that targeted the highest possible efficiency. However, the pump offered a limited operating range or operated inside its recirculation range, which eventually led to poor performance or operation.

Many centrifugal pumps have conventional seals. But magnetic drive and other seal-less options such as hermetically-sealed pumps are gaining in popularity in small- and medium-sized pumping applications. By eliminating mechanical seals, they eliminate the risk of leakage, as well as the maintenance associated with such seals.

However, seal-less units have drawbacks and limitations that limit them to specific services and power ratings. They are usually more expensive. They are unable to handle common challenges such as the presence of particles in liquid. More importantly, magnetic-drive pumps can only be used in relatively low power ratings.

There is a minimum flow for each pump. Below that point, operation is unstable and problematic. As the flow rate approaches minimum flow, vibration increases, and the temperature of the pumped liquid rises. A bypass line or similar solution may be used to recirculate some liquid from the discharge to suction to allow the pump to achieve the minimum required flow.

The pump manufacturer usually provides the minimum continuous required flow rate for each pump. Ample margins should be factored in. For many pumps, this translates into operating above around 60% of BEP flow rate.

The selection of the baseplate and mounting arrangement should be one of the first steps in selecting a pump package. Baseplates are available in a variety of configurations. Those fabricated from strong steel profiles or structures are preferred. Torsional stiffness, rigidity, and flatness are important for a baseplate. Grouting and lifting issues should be addressed properly. Fabricated steel baseplates should be sufficiently strong and heavy duty, properly welded, stress relieved, and machined.

All pumps require some piping: for the lubrication system, seal systems, drains, vents, and any cooling systems. Tubing and flexible hoses are responsible for many failure and reliability issues. Small piping is a better option. Pipes should be properly supported, braced and gusseted to prevent vibration or stress.

Sufficient structural stiffness in the pump and baseplate minimizes pump shaft and driver shaft the misalignment. The configuration of piping should not cause excessive forces and loads on pump nozzles, and should limit misalignment between the pump and driver. Piping should only produce reaction loads within the allowable nozzle load at all operating conditions.

Installation and pre-commissioning of pumps require more attention than is usually exercised. Key requirements for successful commissioning and operation include strong foundations, proper anchor bolts, minimum piping forces, good alignment, and high-quality grouting.

The foundation mass, for example, should be three to five times the mass of the centrifugal pump’s package. Epoxy grout should always be used. Anchor bolts are best with a length/diameter ratio of “11” to “15”. They should have sleeves to prevent entry of grout and accommodate relative thermal growth.

Rotor deflection

For satisfactory performance of seals, the rotor/shaft stiffness should limit total shaft deflection under all operating conditions including the most severe. Deflection should be minimized over the allowable operating range of the pump.

For variable speed pumps, there is more risk of resonance and dynamic excitation. Speed variation should also be considered in any deflection assessment. The maximum deflection should be restricted to a certain level. This deflection limit may be achieved by a combination of large shaft diameter, minimized shaft span (or overhang), and strong casing, including the use of dual volutes or diffusers. For multistage pumps, liquid stiffening effects should be considered as this helps to lower deflection.

As a pump wears and clearances increase, there can be deviations, inefficiencies. and other issues. If internal leakage from the discharge to the suction occurs, the operating curve moves downward. The degraded curve may shift more or less parallel with the original curve. In troubleshooting, a measured operating point too far below the curve indicates it may be time for replacement or refurbishment of the pump.

Degradation can affect the operation of any pump. Maintenance actions should include a review of clearances, and the liquid stiffness of lubricated bearings and bearing bushings. Look out, too, for recirculation – a flow reversal near the inlet or discharge of the centrifugal pump. This flow reversal usually produces cavitational erosion typically starting on the high-pressure side of the impeller vane before proceeding through the low-pressure side. Recirculation can also cause axial movement of the shaft and damage the thrust bearing. ■

Amin Almasi is a Chartered Professional Engineer in Australia and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.